1. Field of the invention
The present invention relates to a dynamic pressure air bearing unit.
2. Description of the prior art
In recent years, dynamic pressure air bearings having spiral grooves have come to be used as a bearing unit for supporting a high-speed rotating body which rotates in the order of tens of thousands of r.p.m.
Methods are known, for example, for the production of a dynamic pressure air bearing unit having the construction as shown in FIG. 6, in which one end portion of a rotating shaft 41 is fitted into a cylindrically shaped bearing member 42 having a closed end, incorporating a spiral groove design with a plurality of threads 43 being formed on either the outer circumferential surface on the one end portion of the rotating shaft 41 or the inner circumferential surface of the bearing member 42, the spiral groove 43 acting as a pump with the rotation of the rotating shaft 41 to impel lubrication air 44 between the rotating shaft 41 and the bearing member 42 toward an inside bottom 42a of the bearing member 42, increasing the air pressure and thus generating a supporting force in the radial direction with the lubrication air 44 acting as a wedge thrusting between the inner circumferential surface of the bearing member 42 and the outer circumferential surface of the rotating shaft 41, while supporting the thrust load by the thrust of the pressure acting on an endmost portion 41a of the rotating shaft 41.
A dynamic pressure air bearing unit of such construction has the advantages of a longer life, lower vibration and little noise as compared with ball bearings, and, making use of such advantages, has come to be used for office peripherals and other equipment, the application including a bearing unit for a polygon mirror for laser printers.
On the other hand, for a conventional fan motor used for example in an electric vacuum cleaner, a DC motor capable of high-speed rotation at 20,000 to 30,000 r.p.m. is used, while the rotor to which a fan is secured is supported by two sets of ball bearings.
In recent years, providing a smaller size, lower noise vacuum cleaner has become a major subject of development, and as the most effective means for reducing the size of a vacuum cleaner, designing a higher speed motor is being studied. As is well known, the output W of a motor is given by: EQU W.varies.L.multidot.D.sup.2 .multidot.N
wherein L is the length of the motor, D the outer diameter, and N the rotating speed. In this case, for example, if N is doubled, the same motor output can be obtained by shortening the length to half or by reducing the outer diameter to 1/1.4.
However, faster motor speed involves various problems such as:
(1) Increased wear and noise of motor brushes. PA0 (2) Shorter life and increased noise of ball bearings.
Since there is a good possibility of solving the problem (1) because of the recent improvement of performance and reduction of costs of brushless motors, the problem (2) has now become the major point yet to be solved.
Accordingly, a dynamic pressure air bearing unit having the aforementioned features may be considered as a replacement for the ball bearings, but the use of the air bearing in a dusty environment like a vacuum cleaner will involve the problem that the infiltration of dust would immediately cause seizure with a resultant damage to the bearing since the bearing clearance .delta. is set at a few .mu.m. Specifically, in the case of the dynamic pressure air bearing as shown in FIG. 6, the dust floating in the vicinity of the bearing opening is drawn into the bearing by the pumping action of the spiral grooves 43, the foreign matter thus caught inside the bearing causing damage and seizure of the bearing.